Mount Nyiragongo, an active volcano in the Virunga Mountains of the Democratic Republic of the Congo, has one of the largest known lava lakes (above).

Photograph by Carsten Peter, National Geographic

On Saturday, September 27, at 11:52 a.m. local time, a cloud of ash, toxic gases, and rock erupted from 10,062-foot Mount Ontake, Japan's second highest volcano, killing at least 36 people and injuring approximately 60 more. The mountain continues to spew poisonous gases, forcing rescue efforts to a standstill, and it's unclear how many hikers are still missing or remain stranded on the mountain. (Related: "Explaining Surprise Eruption of Japanese Volcano.")

For the past 35 years, Mount Ontake has seen little volcanic activity. (Its last significant eruption occurred in 1979, and no one was killed.) Saturday's deadly eruption came as a complete surprise, according to photographer Carsten Peter, a regular contributor to National Geographic, who has spent the past 38 years chasing lava. Peter, 55, has climbed more than a hundred volcanoes, dangled on ropes above bubbling lava lakes, and braved 2,000-degree temperatures (wearing a heat suit) to capture one of the planet's most inaccessible—and dangerous—wonders: the active volcano.

Here Peter offers insights on volcanoes and what likely occurred on Mount Ontake.

What's the allure of climbing volcanoes?

For me, this connection to the inner part of the world is always pretty amazing. This particular volcano in Japan is considered a holy mountain. It's the second highest volcano in Japan after Fuji, and it's a beautiful landscape. It's a very popular site to go hiking, and there is a cableway, so it's easy to reach the top.

There were no signs of a potential eruption, so what happened on Mount Ontake?

I think water entered the mountain somehow, and that's like a boiling pot. The water expands and is blocked by stone and pressure builds, which triggers a nasty eruption called a phreatic eruption.

Why was this eruption so fatal?

It was a beautiful fall day around noon, and there were many people near the top of the mountain. The volcano is not very active and pretty well monitored. There was a little more seismicity, but no real signs of an eruption, so the tourists were completely surprised. This wasn't a big eruption—it was a medium-size one—but the number of people and their proximity to the eruption made it so dangerous.

In an eruption like this, you are engulfed so suddenly with so many gases that just take away your oxygen. It's very difficult to react to that. The question is, What did they really die from? The gases? The ash? They could also have been hit by rocks.

Have you ever had any close calls climbing volcanoes?

Yes, many, but mostly with pyroclastic clouds. These are a very dangerous mixture of gas and ash that are more than 1,000 degrees Fahrenheit. They are unpredictable and move very fast. We had a close call with a pyroclastic cloud on Montserrat [in the West Indies] in 2010. I wanted to photograph a partial collapse of a mountain on the island. I waited for three weeks, and then I ran out of time and went home. Ten days later, a pyroclastic cloud completely wiped out a huge stretch of the landscape where I had been. I wouldn't have had a chance. When I saw how this landscape was transformed, I was very humbled.

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The 1995 eruption of the Soufrière Hills volcano sent lava flows and ash across the island of Montserrat, including the capital city of Plymouth (above). The volcano is still active, and much of the population has left the island.

Photograph by Carsten Peter, National Geographic

Do you need special skills or equipment to assess the danger of a volcano erupting?

Definitely. If you don't know how to approach, then it's easy to get into trouble. It's also very difficult to predict pyroclastic clouds. Special meters to read the gases in the air and gas masks are standard equipment for approaching active volcanoes. Sometimes you need a helmet and goggles, because the ash can blind you. But usually, worldwide authorities close access to active volcanoes. Closures are actually a big problem for me and my work.

Do you need any special skills to climb a volcano?

Most volcanoes are not hard to climb in the technical regard, which makes them a major target for many people to enjoy recreationally. You need skills if you climb into craters, which become quite difficult because there aren't a lot of anchoring possibilities.

Is it safe to climb volcanoes?

It really depends on the status of a volcano and the experience of the tourists. There are different types of volcanoes, too. So you can't say in general if it's safe or not. With Ontake, this seemed to be a very safe volcano, but the tourists didn't have a chance.

Kelley McMillan is a journalist based in Boston. In August, she reported on a team of Nepalese climbers who summited K2.